Oxford Dictionaries offers the definition of an idiom as “a group of words established by usage as having a meaning not deducible from those of the individual words (e.g.,rain cats and dogs, see the light).”
A drop in the bucket.
To beat a dead horse.
Bring home the bacon.
Go against the grain.
These interesting phrases are metaphorical, and usually specific to certain culture of geographical region (which means strictly translating them into other languages is confusing).
The video below from Mental Floss quickly goes through 42 idiom examples in video form, making it a great introduction for your students. But the best part is that it also explains the origin of each phrase–and many of the explanations may surprise you.
42 Idiom Examples And Explanations